1. Field of the Invention
This invention relates to a DC/DC converter which may be used with a large capacity power supply, and, more particularly, to an improved series connected ON/ON or ON/OFF type DC/DC converter in which two one transistor type ON/ON or ON/OFF converters have their primary terminals connected in series circuit and their secondary terminals connected in parallel circuit, so as to increase the output and raise the breakdown voltage.
2. Description of the Prior Art
Examples of known one transistor ON/ON type DC/DC converters are shown in FIGS. 1 and 2. In the converter shown in FIG. 1, an input voltage E.sub.IN is turned "on" and "off" by a transistor Q.sub.1 acting as a switching element, and is applied to primary winding N.sub.11 of transformer T.sub.1 so that a voltage induced at secondary winding N.sub.12 is rectified and smoothed by diodes D.sub.11 and D.sub.12, choke L.sub.1 and smoothing capacitor C.sub.12, whereby DC voltage V.sub.0 is fed to a load RL.
In the one transistor type ON/ON converter, a high backswing voltage is generated between the two terminals of switching element Q.sub.1 by an excited voltage which is stored in the transformer T.sub.1 when switching element Q.sub.1 is turned "off". In order to protect switching element Q.sub.1, therefore, it is customary to equip transformer T.sub.1 with a reset winding N.sub.13, which is connected to diode D.sub.13 during the turning off operation so that its voltage is clamped at the input voltage E.sub.IN thereby to suppress the induced voltage at primary winding N.sub.11 of transformer T.sub.1.
Alternatively, there may be provided a snubber circuit S.sub.1 which comprises a resistor R.sub.1, capacitor C.sub.13 and diode D.sub.14, thereby to effect clamping operation.
A system equipped with reset winding N.sub.13 has a small amount of loss because the excited energy is returned to the input voltage E.sub.IN. If the coupling between reset winding N.sub.13 and primary winding N.sub.11 is insufficient, however, the backswing voltage cannot be clamped sufficiently. On the other hand, a system using a snubber circuit can clamp the backswing voltage at a low level, only if the CR time constant is increased. However, a system using the snubber circuit has a large loss because of the power consumed by resistor R. Thus, in actual practice, it is desirable to use both the above discussed reset winding circuit and the snubber circuit, such as shown in FIG. 1.
As shown in FIG. 2, even in a series connected ON/ON type DC/DC converter using two one-transistor type ON/ON type converters having their primary terminals connected in series and their secondary terminals connected in parallel to increase the output and reduce the breakdown voltage of the switching element, two one-transistor type ON/ON type converters CV.sub.1 and CV.sub.2 are equipped with reset winding N.sub.13 and N.sub.23 and snubber circuits S.sub.1 and S.sub.2. Disadvantageously, this converter is complex and unreliable due to losses in the snubber circuit.
Moreover, disadvantageously, in the circuit of FIG. 2, switching elements Q.sub.1 and Q.sub.2 of the respective converters CV.sub.1 and CV.sub.2 are required to have breakdown voltages which are twice as high as divided input voltages E.sub.1 (i.e. the voltage between two terminals of capacitor C.sub.11) and E.sub.2 (i.e. the voltage between two terminals of capacitor C.sub.21). In case the equivalent input impedances of the respective converters are equal, the switching elements may have a breakdown voltage of E.sub.IN because the equations EQU E.sub.1 =E.sub.2 =E.sub.IN /2
hold. If the equivalent impedances are different, however, the switching elements are required to have a breakdown voltage of no less than E.sub.IN because E.sub.1 =E.sub.2 so that one of the divided input voltages is higher than the other. In other words, the circuit of FIG. 2 has little effect in reduction of the breakdown voltage, and the switching elements are required to have the breakdown voltage which is twice as high as E.sub.IN, if safety is taken into consideration.